Bluetooh: BAP: Fix issue with SRC_ID_VALID flag

The flag was not properly set when the broadcast sink
did not add the source (e.g. if a broadcast assistant
added it).

This also downgrades two LOG_WRN to LOG_DBG as they may
happen without indicating an issue.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>